High levels of thyroid hormone are associated with an increased risk of breast cancer in women, according to a 36 yearlong study of more than four million women published in the European Journal of Endocrinology.

The thyroid is an important gland that releases hormones which control our metabolism; up to 6.7% of Europe's population have undiagnosed thyroid disease. Hyperthyroidism, in which the gland makes too much thyroid hormone, occurs in 51 per 100 000 people per year, and is six times more common in women than in men.

In this study, researchers from Aarhus University Hospital followed 4,177,429 women living in Denmark between 1978 and 2013, accounting for all the women registered and using Denmark's health service over the entire 36 year period. Only women with a first-time diagnosis of thyroid disease and no history of cancer were included.

They found that 80,343 women were diagnosed with hyperthyroidism (too much thyroid hormone) and 61,873 with hypothyroidism (too little thyroid hormone). Hyperthyroid women were 11% more likely to develop breast cancer, whereas those with hypothyroidism were 6% less likely to develop the condition, compared to women in the general population.

"In vitro experiments show that sex hormones such as oestrogen play an important role in the proliferation of breast cancer cells", said lead author of the study Dr Mette Søgaard. "High levels of thyroid hormone levels can have oestrogen-like effects, which may explain why hyperthyroidism is associated with higher risk of breast cancer."

The research team's next step is to investigate whether using thyroid hormones to treat hypothyroidism may also be associated with an increased breast cancer risk.

"Our findings emphasize the importance of raising awareness of breast cancer in women with hyperthyroidism, and further our understanding of this potential risk," said Dr Søgaard.